2.7.4.3 Temporal scope, hydrological response variables and receptor impact variables

In BAs, the potential ecological impacts of coal resource development are assessed in two future time periods – 2013 to 2042 and 2073 to 2102. These are labelled as the short- and long-assessment periods, respectively. Potential ecological changes are quantified in BAs by predicting the state of a select number of receptor impact variables in the short- and long-assessment years. These predictions are made conditional on the values of certain groundwater and surface water statistics that summarise the outputs of numerical model predictions in an interval of time that precedes the assessment year. In all cases these predictions also allow for the possibility that changes in the future may depend on the state of the receptor impact variable in the reference year 2012, and consequently this is also quantified by conditioning on the predicted hydrological conditions in a reference interval that precedes 2012 (see companion submethodology M08 (as listed in Table 1) for receptor impact modelling (Hosack et al., 2018)).

For surface water and groundwater variables predicted from the numerical modelling in the Galilee subregion, the reference assessment interval is defined as the 30 years preceding 2012 (i.e. 1983 to 2012). For surface water variables in the Galilee subregion, the short-assessment interval is defined as the 30 years preceding the short-assessment year (i.e. 2013 to 2042), and similarly the long-assessment interval is defined as the 30 years that precede the long-assessment year (i.e. 2073 to 2102). Maximum groundwater drawdown (metres) is considered across the full 90-year simulation period (2013 to 2102). However, for the uppermost groundwater model layer (Layer 1 – Quaternary alluvium and Cenozoic sediments), the conceptualisation of the boundary conditions means that maximum drawdown does not occur before 2102 (this is explained further in Section 2.6.2.8.1 of companion product 2.6.2 for the Galilee subregion (Peeters et al., 2018)). Thus, for the purposes of the receptor impact modelling workshop, the time to maximum drawdown was effectively considered fixed at 2102.

In BAs, choices about receptor impact variables must balance the project’s operational constraints with the objectives of the assessment and the expectations of the community (companion submethodology M10 (as listed in Table 1) for analysing impacts and risks (Henderson et al., 2018)). This choice is guided by selection criteria that acknowledge the potential for complex direct and indirect effects within perturbed ecosystems, and the need to keep the expert elicitation of receptor impact models tractable and achievable (companion submethodology M08 (as listed in Table 1) for receptor impact modelling (Hosack et. al., 2018)).

For the streams landscape groups, the qualitative modelling workshop identified mine dewatering of shallow and deep groundwater, mine interception of surface water through subsidence and fracturing, and mine interception of precipitation and its effect on the recharge of confined groundwater and surface water flows as the hydrological factors that were thought to: (i) be instrumental in maintaining and shaping the ecosystem’s components and processes, and the ecological values provided by the ecosystem and, (ii) have the potential to change due to coal resource development (Figure 20). All of the ecological components and processes represented in the qualitative model are potential receptor impact variables and most of these are predicted to vary as the hydrological factors vary either individually or in combination (Table 12 and Table 13). Further information about the surface water and groundwater modelling approaches used to simulate the hydrological effects of the main mining processes identified above is discussed in companion product 2.6.1 (Karim et al., 2018) and companion product 2.6.2 (Peeters et al., 2018) for the Galilee subregion. For example, the surface water modelling approach to address mine interception of rainfall and overland flow is outlined in Section 2.6.1.3 of Karim et al. (2018).

At the initial qualitative modelling workshop, the following receptor impact variables were chosen: riparian trees, and algae and/or invertebrates. The second receptor impact variable was specified further on the initial day of the second receptor impact modelling workshop in Brisbane in October 2016. The decision was taken to use a species of mayfly (order Ephemeroptera) in the genus Offadens of the family Baetidae (Webb and Suter, 2011). The identification of the genus is based on expert feedback from Phil Suter, La Trobe University, Wodonga, Victoria (Dr Phil Suter, 26 Oct 2016, pers. comm.).

The mayfly in the genus Offadens was chosen as the invertebrate receptor impact variable for a number of reasons, specifically: (i) data were available for the species from sampling locations in the upper Burdekin river basin (i.e. the Cape and Campaspe rivers), north-east of the zone of potential hydrological change (Blanchette, 2012); and (ii) the species requires fast-flowing streams. It was deemed that a suitable receptor impact variable needed to respond to flow and to be impacted by water drying up.

The species can recolonise within 1 to 2 days of flows but is challenged by low-flow periods of greater than 14 days. Water depth in riffles is assumed to be more than 2 cm. Turbidity is not a driver for this species. There is no legacy effect in terms of how mayflies respond to changing flow conditions.

The hydrological factors identified by the participants in the qualitative modelling workshops have been interpreted as a set of hydrological response variables. The hydrological response variables are summary statistics that: (i) reflect these hydrological factors, and (ii) can be extracted from BA’s numerical surface water and groundwater models during the reference-, short- and long-assessment intervals defined previously. The hydrological factors and associated hydrological response variables for the streams landscape groups are summarised in Table 14. The precise definition of the receptor impact variable, typically a species or group of species represented by a qualitative model node, was determined during the receptor impact modelling workshop.

Using this interpretation of the hydrological response variables, and the receptor impact variable definitions derived during the receptor impact modelling workshop, the relationships identified in the qualitative modelling workshop were formalised into two receptor impact models (Table 15).

Table 14 Summary of hydrological response variables used in receptor impact models for the streams landscape groups in the Galilee subregion zone of potential hydrological change, and the corresponding variables for the signed directed graphs

Table 15 Summary of the two receptor impact models developed for the streams landscape groups in the Galilee subregion zone of potential hydrological change

Hydrological response variables are defined in Table 14.